How many letters does the top answer for “a particularly baffling problem that is said to have a correct solution” have?

The most common answer is mystifier (9 letters).

Crossword clue: a particularly baffling problem that is said to have a correct solution

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

mystifier 9 letters wn-def

a particularly baffling problem that is said to have a correct solution

puzzler 7 letters wn-def

a particularly baffling problem that is said to have a correct solution